In an internal combustion engine including a purifying catalyst in an exhaust passage, feedback control of an air-fuel ratio is performed so as to maintain the air-fuel ratio in the vicinity of a theoretical air-fuel ratio where purification efficiency of the catalyst is high. Japanese Patent Application Laid-Open No. 10-288075(Patent Document 1) discloses an air-fuel ratio control apparatus that performs high-accuracy feedback control by using an air-fuel ratio sensor capable of detecting an air-fuel ratio over a wide range of operation.
However, although the apparatus described in Patent Document 1 performs the feedback control to a theoretical air-fuel ratio, the control is switched to a feedforward control in a rich air-fuel ratio range where a fuel injection amount is made that is larger than an amount equivalent to the theoretical air-fuel ratio at the time of acceleration or the like. This disadvantageously results in larger fluctuations with respect to a target value of the air-fuel ratio in the rich air-fuel ratio range, which causes fluctuations in output performance.